You have peaked my interest sir.
So the weakest part of the module is the hook, but that's ok because hooks really should be customized to the party. What hooks one character really isn't a hook for another. Think of the seen in Star Wars where Luke is trying to convince Han to join the party and he first tries as a hook, "We've got to save the princess, their planning to execute her", and then "She's beautiful", and finally, "She's rich", which gets Hans attention.
For a one shot, you have some freedom that you don't have in a campaign. I'd suggest two things:
a) Start in media res. The campaign start is the only time you'll really get a chance to set the stage as a director, your only chance to say what the characters have done and are doing, so you might as well use it. Have the characters meet in some way while they are already on their way to Barovia. Focus on one character initially, who is in some minor difficulty - late for a carriage, trying to get a room in an inn during a thunderstorm only to find all the rooms are taken, and resolve this minor difficulty by having the character find all the other PCs are travellers who are there ahead of him. Take the time to make the players interact with each other IC. The new PC is trying to squeeze into an already crowded carriage, the inn keeper is trying to convince a PC to let this new PC share a bed with them, or whatever you come up with. The hidden point to all of this is you've already set the stage with, "You've all already bitten the hook" That resolves the problem with the weak hook nicely. In this version of the hook, not every PC needs to know every other PC, which is a nice state to have because introductions are a great intra-party oppurtunity. Additionally, and this is more work for you, each group of PCs can have their own hook - their own reason they are travelling to Barovia. One of them should be the standard letter provided by the text, but customize the hooks to the characters.
b) So keep in mind that the best stories in an RPG involve the PCs being the protagonists in every way. It's a perfectly acceptable hook that Strahd believes a female NPC is his lost love (or even that this is true!), the reincarnation of his foe who is prophisied to defeat him, or his long lost brother, or anything else of that sort. Tightly integrate the PC into the story.
c) The Tarot reading is a very critical scene and it needs to happen to set the story and it needs to happen early. It's quite possible to miss the gypsy camp in Barovia or that the players will deliberately skip it fearing a trap. Don't wait for the scene to occur in Barovia, and don't let them miss the scene. Have the gypsy fortune teller encountered as near to the beginning of the story as you can, and above all - stack the deck. Don't leave the results up to random chance. Deal yourself a few hands before the session and decide what you think the most interesting results and most applicable results for the party would be. If necessary, invent new meanings that you think are more suitable to the story you want to tell with these PCs. Then put the cards behind your DM screen, and arrange them according to the how the players sit, and stage the scene with the fortune teller on the journey to Barovia in the way that makes the most sense to you. I often have her as a solitary beggar woman. Hopefully your players play along, if not, set an oppurtunity for a fortune telling later - with the gypsy camp of the text being your last recourse.
Ok, so the other problem that I have with the module is a typical Hickman problem - he's very tightly focused and he pretty much skips over any transitions or leaves them up to the DM. Just as in I3 Pyramid, the wilderness map of the region surrounding the dungeon is too small.
d) Create a new map around Castle Ravenloft, suitable for a sovereign Barony with maybe a second town (Zarovich?) and a few villages, and room for other stuff if you want or need to add it. Read the section of Dracula near the beginnning where Jonathan describes his journey through the Transylvanian countryside to see the mysterious Dracula. That's what you are going for in this section. You don't want to linger to much on it. Don't make them play out the journey in hourly detail, but do give them the sense of the scale and the movement from civilization to something that is older and less than that.
e) Add an encounter to the journey where the party is ambushed by 'bandits', ideally something they overmatch - 1st-3rd level warriors for example. Probably the PCs will just start slaughtering bandits because they've attacked them (or attempted to), which is fine. If not, that's good too - the PCs have a head start on their role play. If the bandits start to take a lot of casualties, have them run away. If they escape great, you've got a reoccuring NPC. If not, that's good too. When the PC's loot the bodies, have them discover that they are all tatood with a particular symbol, or are carrying a particular symbol that seems like a holy symbol but isn't. The PC's should later discover that this symbol is associated with anti-Strahd freedom fighters who want to overthrow his rule. The freedom fighters have learned the PCs are Strahd's invited guests, and in a bid to thwart him and a bit of misunderstanding, have attacked the PCs thinking that they are Strahd's allies. This sets the stage for intrigue in town.
I am trying to think of ways to add RP and intrigue value in town. Maybe some Strahd spies?
f) So, you need to add some more detail to the town. I'd suggest creating about two to three dozen additional stand in townsfolk. Most of these should be very low level. Basically, there are three positions you can take if you are one of the citizens (or cattle) of Barovia. Either you can side with Strahd hoping to gain his favor and get him to bite someone else -these are his allies that he can use against the party. Or you can decide that the best course is to keep your head down and draw no attention to yourself. These are people the party can recruit. Or you can be one of the few, the proud, the stupid, that decide to resist his evil. As a subgroup of this last category, there are actually going to be a few hopelessly overmatched undead slayers, vigillates, and authentic survivors who've learned how to survive in Barovia. These are potential allies, paramours, and NPCs that you think will draw sympathy, admiration, and even possibly a crush on your fictional character (which is one of the heights of compliments you can be paid as a DM). You may or may not get to use any of these characters, but you need to have them on standby in case the plot moves in an unexpected direction - for example the PCs might want to try to rally the townsfolk to take up pitchforks and torches and storm the castle at some point.
The initial set up with the townsfolk refusing to bury the body and the funeral is just awesome sauce. Go with that. Bring Stahd into play after the funeral with his initial sort of wake up call. It's worth it to establish that there is at least one temple in the area that is sacred ground (consecrated in 3e terms) where the PCs are at an advantage. When I was first a player the DM chased the party to the Temple, and I find this works pretty well as a scene and try to replicate it. You can have one of the vigilantes try to help the party to clue them in, and so forth. The important thing is to get Strahd some screen time, even if he doesn't directly attack the players. Score yourself as a DM by how many scenes Strahd gets to have interacting with the PCs. Disguise him as another character. Have him talk to the PC's from a distance. Have him engage in conversation with isolated PCs. Have him toy with the party or individuals. Never have him stand up to and fight the whole party. Strahd never risks his life. He has all the time in the world - he reginerates, he has amazing mobility, and he's a high level spellcaster with a broad spell selection and the ability to plan. If you are using energy drain, the PCs are dead. They almost can't win, which is why I recommend you don't and replace the energy drain with a more vampiric blood drain attack. You score a point as a DM every time you put Strahd onstage and get to explore him. The exact methodology here is going to depend on the hooks you had.
At some point this plot is going to fork past where I can give you specific advice, both because you've hopefully come up with your own cool hooks and the players will have their own agenda.
And some involvement with that ghost train that goes from the graveyard to the tower. That element is cool but what can I do with it? One of the ghosts is.. someone Ireena knows and .. they can talk? Like the book Dante's Inferno, Dante can pull a soul out and talk to them and when they go back its like they never left..
I've never really done anything with the ghost train, but your line of thought sounds sound. Technically, the ghosts are phantasms and not ghosts but you might want to do something creative with the Pathfinder haunt rules while you are doing Bavaria. Particularly if you've decided to have the PCs be reincarnated, you can have them be haunted just about everywhere. It's a good time to also mention that I favor using the 3e Ravenloft rules for fear, horror, and madness.
I'm looking for something for them to do outside the castle. Like I'm thinking of maybe putting some element or treasure outside the castle they'd need to get. I thought maybe put the sunsword outside or the holy symbol that there could be hints to where it is or something.
Hints are definately in order. You can use the resistance fighters as means of feeding clues about the existance of the sunsword and so forth once (and if) the PCs gain the resistance fighters trust. A mini-dungeon might be ok, but you'd need to explain a) why Strahd hasn't raided and destroyed it and b) why it wasn't breached by good guys sometime in the last centuries. Keep in mind Strahd is a genious and is very much capable of charming his foes and getting them to do his dirty work if he can't.
Anyway, remove the aforementioned green slime and gargoyles that are there just to kill any PCs that try to run away, and reduce the size of the crypts to something more managable. I made a variaty of other small tweaks as well, but I dont' recall them off the top of my head. I'll see if I can find my notes at some point.
Since Ravenloft was pre-3.5, I'm wondering how to alter it to account for pathfinder classes. I've found something called Mistfinder that gives an updated Strahd as a level 15? like paladin/necromancer or something and they make some cool tweaks to the paladin class.
I don't think you need to update it for anything specific to Pathfinder. Gunslingers will fit in fine to the gothic setting if you go that way. The real challenge is going to be balancing things right during the conversion to 3.X. Do NOT up Strahd to 15th level, unless you plan on getting the Pc's up to like 11th level - which is going to wreck the balance of everything else and make it that much harder to convert. The ideal balance is that the party is just below a point where they can turn Strahd (with any reliability). They shouldn't have as much HD as Strahd, but they shouldn't be so far behind that he completely outmatches them. I've never run Ravenloft as a 3.X module, so I can't help with the conversion. Somewhere I have a full conversion for I3 Pyramid though.
I mean "you wake up and are surrounded by this fog that rolled in over night. You don't know what happened but when you walk into the fog you start choking." Its easy to drop in.
That's too easy. Don't yank your players around like that much or you won't have any players. Many players have a very low tolerance for that sort of railroading.